A broadband wireless access system to transmit a packet to user equipment (UE) based on an IP-based Voice over Internet Protocol (VoIP) service is described below. VoIP traffic is created with a fixed size having a fixed period within a VoIP codec. VoIP communication may be classified into a talk-spurt period and a silence period. During the talk-spurt period, a speech mode is maintained between users. During the silence period, the user listens to the other party without talking to the other party. The silence period may occupy 50% or more of a general call session.
Therefore, in order to allocate different amounts of bandwidths to the talk-spurt period and the silence period, a variety of voice codecs may be used. A representative example of the voice codec is an adaptive multi-rate (AMR) for use in a Global System for Mobile communication (GSM) and a Universal Mobile Telecommunications System (UMTS).
Voice data is not generated during the silence period. If the bandwidth is allocated to the silence period, radio resources may be wasted. In order to prevent this problem, VoIP supports a silence suppression scheme. According to the silence suppression scheme, a vocoder for generating the VoIP traffic does not generate traffic data. However, the vocoder periodically generates comfort noise to inform the other user that the call is still ongoing. For example, the vocoder based on the above-mentioned AMR codec generates a fixed-sized packet at intervals of 20 ms in the talk-spurt period, and generates the comfort noise at intervals of 160 ms in the silence period.
In order to perform resource allocation of traffic data, which has a fixed-sized constant period such as a VoIP, a Node-B or base station (BS) may fixedly assign a predetermined region to a specific UE. For example, the BS assigns the region as large as the initially-defined size to the UE capable of supporting the VoIP service. The BS may also inform the UE of the assigned resource region information using a control channel or a control message such as a UL-MAP or a DL-MAP. Accordingly, the control channel or the control message, which is initially transmitted, may also include period information of the next allocation region.
From the next period, the BS may successively assign a corresponding region without any specified notification associated with the above region which has been notified to the mobile station (MS), using the initially-transmitted control channel or the initially-transmitted control message. Therefore, the MS transmits the VoIP packet to the assigned region using region information which has been initially assigned on the map and transmits the VoIP packet from the next period to the same region using period information.
For example, when the frame length is set to 5 ms in consideration of the VoIP service and a frame period assigned to the MS for VoIP packet transmission is set to 4 frames, the frame period assigned to the MS for the VoIP packet transmission may be changed to another according to service characteristics. Specifically, if the same VoIP service is used, the frame period assigned for VoIP packet transmission may be defined in different ways according to individual elements such as system characteristic and VoIP service status. For example, the system characteristic may be changed according to the frame length and the VoIP service status may be either the talk-spurt period or the silence period.
The BS located at an initial frame informs the MS of the allocation region information to transmit the VoIP packet via the UL-MAP. If the BS is located at a fourth or eighth frame corresponding to each period, it does not inform the region information via the UL-MAP and assigns only the region for VoIP packet transmission.
In this case, the period assigned for the VoIP packet transmission may be 4 frames or 20 ms.
The MS stores region allocation information contained in the UL-MAP message received from the initial frame. Therefore, the MS can transmit the VoIP packet via a corresponding region although the UL-MAP message is not additionally received from the fourth and eighth frames corresponding to the resource allocation period. Accordingly, the BS is fixed to a single VoIP connection due to the VoIP traffic characteristic and can persistently assign resources to this VoIP connection.
If a control message for assigning a new transmission region is lost in a wired or wireless downlink, the conventional system has a disadvantage in that individual transmission region information recognized by the MS and the BS is unavoidably different from each other. Therefore, in case of the downlink, the MS is unable to receive the VoIP packet from the BS.
In case of the uplink, although the MS transmits the VoIP packet via the second transmission region, the BS may assign the second transmission region to another MS, resulting in unexpected problems. In other words, if the MS transmits the VoIP packet via the second transmission region, a packet transmission by another MS may fail. If the system uses an incremental redundancy (HARQ-IR) scheme, the number of retransmission will be increased, resulting in unexpected problems.